We are investigating the chemical intermediates involved in transferring electrons at high reduction potentials and in storing the oxidizing equivalents needed to yield O 2. Manganese is involved in the water-oxidizing enzyme, and we are using x-ray absorption spectroscopy to determine the oxidation states, ligation, and local structure around the Mn. Together with studies using EPR spectroscopy, these studies have verified the direct participation of Mn oxidation state changes in the water-oxidizing process. This proposal aims to characterize further the structure of the quartet of Mn atoms involved, the essential cofactors calcium and chloride, inhibitors and protein ligands through the use of X-ray absorption spectroscopy.